Abstract: The effects of Zn+Mg content and Zn/Mg ratio in the weld metal of Al-5Zn-1.6Mg alloy on the behaviour of joints,and the stress corrosion resistance in particular were investigated by means of varying the filler metal composition with TIG technique and the post-weld heat treatment.Results show that stress corrosion resistance not only relates with the Zn+Mg content,but also depends upon the Zn/Mg ratio in the weld metal.When the Zn/Mg ratio is around 2.7,the greatest resistance to stress corrosion cracking can be obtained provided that the Zn+Mg content is kept constant.Of this conclusion designers of some welded marine structures have taken advantage.Experience in preventing weld cracking in complex structures with various types of joints is also described,the base metal being Al-5Zn-1.6Mg alloy which is susceptible to hot cracking.The strenghth of welded structures in T6 condition was tested.Some of the structures have been immersed in sea-water and others have been exposed to marine atmosphere for four years without any fracture due to stress corrosiom.